It is highly desirable to be able to determine at as early a stage as possible the amount of hydrogen sulphide in oil and gas deposits in the earth formations surrounding a wellbore, since the amount of hydrogen sulphide can seriously impact the economic value of the deposits, and affect the composition (and therefore the cost) of the metalwork used in the extraction of the deposits from the formations. Additionally, because hydrogen sulphide is toxic in even relatively low concentrations, the hydrogen sulphide content of the deposits has an important bearing on the health, safety and environmental aspects of their extraction.
Several methods and apparatuses for the measurement of the hydrogen sulphide content of wellbore fluids are described in International Application No. WO 01/63094 (now granted as UK Patent No. 2 395 631). Among these are a method and apparatus based on an electrochemical sensor in which the current created by a redox reaction involving the hydrogen sulphide is measured. More specifically, the sensor comprises a reaction chamber or cell containing a precursor or catalyst (hereinafter referred to simply as a precursor) in an aqueous reaction solution, the walls of the chamber including a gas permeable membrane over which the wellbore fluids flow and through which hydrogen sulphide in the wellbore fluids diffuses into the reaction chamber to initiate the redox reaction, at the surface of an electrode controlled at certain voltage.
However, as the search for hydrocarbons is extended, wellbores are becoming deeper, so that the environment, in which electrochemical sensors are required to operate, is becoming increasingly hostile. Typically, the sensors need to be able to operate at temperatures of up to 200 degrees Celsius and pressures of up to 20,000 psi.
It is an object of the present invention to provide new electrochemical sensors of the type in which the current created by a redox reaction involving the hydrogen sulphide is measured, and which are suitable for use in severe borehole environments.